2x2 Table Disease No Disease Total Exposed a b ab Not exposed c d cd Total ac

2x2 table disease no disease total exposed a b ab not

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2x2 Table Disease No Disease Total Exposed a b a+b Not exposed c d c+d Total a+c b+d a+b+c+d
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Copyright © 2018 Cognella, Inc. Cross-sectional/Prevalence Studies Across the rows: a + b = total number of individuals exposed c + d = total number of individuals not exposed
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Copyright © 2018 Cognella, Inc. 2x2 Table Disease No Disease Total Exposed a b a+b Not exposed c d c+d Total a+c b+d a+b+c+d
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Copyright © 2018 Cognella, Inc. Cross-sectional/Prevalence Studies Down the columns a + c = the number of individuals with disease b + d = the number of individuals without disease
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Copyright © 2018 Cognella, Inc. 2x2 Table High BP Normal BP Total High salt diet 400 (a) 200 (b) 600(a+b) Low salt diet 100 (c) 300 (d) 400(c+d) Total 500 (a+c) 500(b+d) 1,000 (a+b+c+d)
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Copyright © 2018 Cognella, Inc. Prevalence calculation The prevalence of high blood pressure in the whole population is: (a+c)/(a+b+c+d) = 500/1000 or .50 or 50% The prevalence of high blood pressure in those who eat a high salt diet is: a/(a+b) = 400/600 = .67 or 67% The prevalence of high blood pressure in those who eat a low salt diet is: c/(c+d) = 100/400 = .25 or 25%
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Copyright © 2018 Cognella, Inc. Prevalence Ratio Calculation Compare the two groups by calculating a prevalence ratio Prevalence ratio = [a/(a+b)]/[c/(c+d)] Divide the prevalence in the high salt group (exposed) to the prevalence in the low salt group (unexposed) 0.67/0.25 = 2.68 The prevalence of high blood pressure is 2.68 times higher among those who eat a high salt diet as compared to those who eat a low salt diet
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Copyright © 2018 Cognella, Inc. Prevalence Ratio Calculation If the prevalence ratio = 1, we conclude the prevalence in the two groups is the same If the prevalence ratio is > 1, we conclude the prevalence in the exposed group is higher than the unexposed group If the prevalence ratio is < 1, we conclude the prevalence in the exposed group is lower than the unexposed group
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Copyright © 2018 Cognella, Inc. Cross-sectional/Prevalence Studies Unable to establish sequence of events Cannot confirm exposure occurred before outcome Cannot test a time-related factor because all measures are made at the same time Infeasible for studying rare conditions
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